5Embedded SQL

5.1 Using Host Variables

Use host variables to pass data and status information to your program from the database, and to pass data to the database.

5.1.1 Output Versus Input Host Variables

Depending on how they are used, host variables are called output or input host variables. Host variables in the INTO clause of a SELECT or FETCH statement are called output host variables because they hold column values output by Oracle. Oracle assigns the column values to corresponding output host variables in the INTO clause.

All other host variables in a SQL statement are called input host variables because your program inputs their values to Oracle. For example, you use input host variables in the VALUES clause of an INSERT statement and in the SET clause of an UPDATE statement. They are also used in the WHERE, HAVING, and FOR clauses. In fact, input host variables can appear in a SQL statement wherever a value or expression is allowed.

You cannot use input host variables to supply SQL keywords or the names of database objects. Thus, you cannot use input host variables in data definition statements (sometimes called DDL) such as ALTER, CREATE, and DROP. In the following example, the DROP TABLE statement is invalid:

Notice that the input host variables in the VALUES clause of the INSERT statement are prefixed with colons.

5.2 Using Indicator Variables

You can associate any host variable with an optional indicator variable. Each time the host variable is used in a SQL statement, a result code is stored in its associated indicator variable. Thus, indicator variables let you monitor host variables.

You use indicator variables in the VALUES or SET clause to assign NULLs to input host variables and in the INTO clause to detect NULLs or truncated values in output host variables.

5.2.1 Input Variables

For input host variables, the values your program can assign to an indicator variable have the following meanings:

Variable

Meaning

-1

Oracle will assign a NULL to the column, ignoring the value of the host variable.

>= 0

Oracle will assigns the value of the host variable to the column.

5.2.2Output Variables

For output host variables, the values Oracle can assign to an indicator variable have the following meanings:

Variable

Meaning

-2

Oracle assigned a truncated column value to the host variable, but could not assign the original length of the column value to the indicator variable because the number was too large.

-1

The column value is NULL, so the value of the host variable is indeterminate.

0

Oracle assigned an intact column value to the host variable.

> 0

Oracle assigned a truncated column value to the host variable, assigned the original column length (expressed in characters, instead of bytes, for multibyte Globalization Support host variables) to the indicator variable, and set SQLCODE in the SQLCA to zero.

Remember, an indicator variable must be declared as a 2-byte integer and, in SQL statements, must be prefixed with a colon and appended to its host variable (unless you use the keyword INDICATOR).

5.2.3 Inserting NULLs

You can use indicator variables to insert NULLs. Before the insert, for each column you want to be NULL, set the appropriate indicator variable to -1, as shown in the following example:

SQLCODE in the SQLCA is set to zero indicating that Oracle executed the statement without detecting an error or exception.

Without an indicator variable there is no way to know whether or not a NULL was returned. The value of the host variable is undefined. If you do not use indicator variables, set the precompiler option UNSAFE_NULL=YES. Oracle therefore advises that UNSAFE_NULL=YES only be used to upgrade existing programs and that indicator variables be used for all new programs.

When UNSAFE_NULL=NO, if you select or fetch NULLs into a host variable that lacks an indicator variable, Oracle issues an error message.

The next example shows how to compare values for equality when some of them might be NULLs:

EXEC SQL SELECT ENAME, SAL
INTO :EMP_NAME, :SALARY
FROM EMP
WHERE (COMM = :COMMISSION) OR ((COMM IS NULL) AND
(:COMMISSION:IND-COMM IS NULL))
END-EXEC.

5.2.7 Fetching Truncated Values

If a value is truncated when fetched into a host variable, no error is generated. In all cases a WARNING will be signalled (see "Warning Flags"). if an indicator variable is used with a character string, when a value is truncated, the indicator variable is set to the length of the value in the database. Note that no warning is flagged if a numeric value is truncated.

5.3 The Basic SQL Statements

Executable SQL statements let you query, manipulate, and control Oracle data and create, define, and maintain Oracle objects such as tables, views, and indexes. This chapter focuses on statements which manipulate data in database tables (sometimes called DML) and cursor control statements.

The following SQL statements let you query and manipulate Oracle data:

SQL Statements

Description

SELECT

Returns rows from one or more tables.

INSERT

Adds new rows to a table.

UPDATE

Modifies rows in a table.

DELETE

Removes rows from a table.

When executing a data manipulation statement such as INSERT, UPDATE, or DELETE, you want to know how many rows have been updated as well as whether it succeeded. To find out, you simply check the SQLCA. (Executing any SQL statement sets the SQLCA variables.) You can check in the following two ways:

Implicit checking with the WHENEVER statement

Explicit checking of SQLCA variables

Alternatively, when MODE={ANSI | ANSI14}, you can check the status variable SQLSTATE or SQLCODE. For more information, see "ANSI SQLSTATE Variable".

When executing a SELECT statement (query), however, you must also deal with the rows of data it returns. Queries can be classified as follows:

queries that return no rows (that is, merely check for existence)

queries that return only one row

queries that return more than one row

Queries that return more than one row require an explicitly declared cursor or cursor variable. The following embedded SQL statements let you define and control an explicit cursor:

SQL Statements

Description

DECLARE

Names the cursor and associates it with a query.

OPEN

Executes the query and identifies the active set.

FETCH

Advances the cursor and retrieves each row in the active set, one by one.

CLOSE

Disables the cursor (the active set becomes undefined.)

In the coming sections, first you learn how to code INSERT, UPDATE, DELETE, and single-row SELECT statements. Then, you progress to multirow SELECT statements. For a detailed discussion of each statement and its clauses, see the Oracle Database SQL Reference.

5.3.1 Selecting Rows

Querying the database is a common SQL operation. To issue a query you use the SELECT statement. In the following example, you query the EMP table:

The column names and expressions following the keyword SELECT make up the select list. The select list in our example contains three items. Under the conditions specified in the WHERE clause (and following clauses, if present), Oracle returns column values to the host variables in the INTO clause. The number of items in the select list should equal the number of host variables in the INTO clause, so there is a place to store every returned value.

In the simplest case, when a query returns one row, its form is that shown in the last example (in which EMPNO is a unique key). However, if a query can return more than one row, you must fetch the rows using a cursor or select them into a host array.

If a query is written to return only one row but might actually return several rows, the result depends on how you specify the option SELECT_ERROR. When SELECT_ERROR=YES (the default), Oracle issues an message if more than one row is returned.

When SELECT_ERROR=NO, a row is returned and Oracle generates no error.

5.3.1.1 Available Clauses

You can use all of the following standard SQL clauses in your SELECT statements: INTO, FROM, WHERE, CONNECT BY, START WITH, GROUP BY, HAVING, ORDER BY, and FOR UPDATE OF.

5.3.2 Inserting Rows

You use the INSERT statement to add rows to a table or view. In the following example, you add a row to the EMP table:

Each column you specify in the column list must belong to the table named in the INTO clause. The VALUES clause specifies the row of values to be inserted. The values can be those of constants, host variables, SQL expressions, or pseudocolumns, such as USER and SYSDATE.

The number of values in the VALUES clause must equal the number of names in the column list. You can omit the column list if the VALUES clause contains a value for each column in the table in the same order they were defined by CREATE TABLE, but this is not recommended because a table's definition can change.

5.3.3 DML Returning Clause

The INSERT, UPDATE, and DELETE statements can have an optional DML returning clause which returns column value expressions expr, into host variables hv, with host indicator variables iv. The returning clause has this syntax:

The number of expressions must equal the number of host variables. This clause eliminates the need for selecting the rows after an INSERT or UPDATE, and before a DELETE when you need to record that information for your application. The DML returning clause eliminates inefficient network round-trips, extra processing, and server memory. You will also note, for example, when a trigger inserts default values or a primary key value.

The returning_clause is not allowed with a subquery. It is only allowed after the VALUES clause.

5.3.6 Deleting Rows

You use the DELETE statement to remove rows from a table or view. In the following example, you delete all employees in a given department from the EMP table:

EXEC SQL DELETE FROM EMP
WHERE DEPTNO = :DEPT-NUMBER
END-EXEC.

You can use the optional WHERE clause to specify the condition under which rows are deleted.

5.3.7 Using the WHERE Clause

You use the WHERE clause to select, update, or delete only those rows in a table or view that meet your search condition. The WHERE-clause search condition is a Boolean expression, which can include scalar host variables, host arrays (not in SELECT statements), and subqueries.

If you omit the WHERE clause, all rows in the table or view are processed. If you omit the WHERE clause in an UPDATE or DELETE statement, Oracle sets SQLWARN(5) in the SQLCA to 'W' to warn that all rows were processed.

5.4 Cursors

To process a SQL statement, Oracle opens a work area called a private SQL area. The private SQL area stores information needed to execute the SQL statement. An identifier called a cursor lets you name a SQL statement, access the information in its private SQL area, and, to some extent, control its processing.

For static SQL statements, there are two types of cursors: implicit and explicit. Oracle implicitly declares a cursor for all data definition and data manipulation statements, including SELECT statements that use the INTO clause.

The set of rows retrieved is called the results set; its size depends on how many rows meet the query search condition. You use an explicit cursor to identify the row currently being processed, which is called the current row.

When a query returns multiple rows, you can explicitly define a cursor to

Process beyond the first row returned by the query

Keep track of which row is currently being processed

A cursor identifies the current row in the set of rows returned by the query. This allows your program to process the rows one at a time. The following statements let you define and manipulate a cursor:

DECLARE

OPEN

FETCH

CLOSE

First you use the DECLARE statement (more precisely, a directive) to name the cursor and associate it with a query.

The OPEN statement executes the query and identifies all the rows that meet the query search condition. These rows form a set called the active set of the cursor. After opening the cursor, you can use it to retrieve the rows returned by its associated query.

Rows of the active set are retrieved one by one (unless you use host arrays). You use a FETCH statement to retrieve the current row in the active set. You can execute FETCH repeatedly until all rows have been retrieved.

When done fetching rows from the active set, you disable the cursor with a CLOSE statement, and the active set becomes undefined.

5.4.1 Declaring a Cursor

You use the DECLARE statement to define a cursor by giving it a name, as the following example shows:

The cursor name is an identifier used by the precompiler, not a host or program variable, and should not be declared in a COBOL statement. Therefore, cursor names cannot be passed from one precompilation unit to another. Cursor names cannot be hyphenated. They can be any length, but only the first 31 characters are significant. For ANSI compatibility, use cursor names no longer than 18 characters.

If MODE is specified at a higher level than CLOSE_ON_COMMIT, then MODE takes precedence. The defaults are MODE=ORACLE and CLOSE_ON_COMMIT=NO. If you specify MODE=ANSI then any cursors not using the WITH HOLD clause will be closed on COMMIT. The application will run more slowly because cursors are closed and re-opened many times. Setting CLOSE_ON_COMMIT=NO when MODE=ANSI results in performance improvement. To see how macro options such as MODE affect micro options such as CLOSE_ON_COMMIT, see "Precedence of Option Values".

The SELECT statement associated with the cursor cannot include an INTO clause. Rather, the INTO clause and list of output host variables are part of the FETCH statement.

Because it is declarative, the DECLARE statement must physically (not just logically) precede all other SQL statements referencing the cursor. That is, forward references to the cursor are not allowed. In the following example, the OPEN statement is misplaced:

The cursor control statements (DECLARE, OPEN, FETCH, CLOSE) must all occur within the same precompiled unit. For example, you cannot declare a cursor in source file A.PCO, then open it in source file B.PCO.

Your host program can declare as many cursors as it needs. However, in a given file, every DECLARE statement must be unique. That is, you cannot declare two cursors with the same name in one precompilation unit, even across blocks or procedures, because the scope of a cursor is global within a file.

For users of MODE=ANSI or CLOSE_ON_COMMIT=YES, the WITH HOLD clause can be used in a DECLARE section to override the behavior defined by the two options. With these options set, the behavior will be for all cursors to be closed when a COMMIT is issued. This can have performance implications due to the overhead of re-opening the cursor to continue processing. The careful use of WITH HOLD can speed up programs that need to conform to the ANSI standard for precompilers in most respects.

5.4.2 Opening a Cursor

Use the OPEN statement to execute the query and identify the active set. In the following example, a cursor named EMPCURSOR is opened.

EXEC SQL OPEN EMPCURSOR END-EXEC.

OPEN positions the cursor just before the first row of the active set. However, none of the rows is actually retrieved at this point. That will be done by the FETCH statement.

Once you open a cursor, the query's input host variables are not reexamined until you reopen the cursor. Thus, the active set does not change. To change the active set, you must reopen the cursor.

The amount of work done by OPEN depends on the values of three precompiler options: HOLD_CURSOR, RELEASE_CURSOR, and MAXOPENCURSORS. For more information, see their alphabetized entries in "Using Pro*COBOL Precompiler Options".

5.4.3 Fetching from a Cursor

You use the FETCH statement to retrieve rows from the active set and specify the output host variables that will contain the results. Recall that the SELECT statement associated with the cursor cannot include an INTO clause. Rather, the INTO clause and list of output host variables are part of the FETCH statement. In the following example, you fetch into three host variables:

The cursor must have been previously declared and opened. The first time you execute FETCH, the cursor moves from before the first row in the active set to the first row. This row becomes the current row. Each subsequent execution of FETCH advances the cursor to the next row in the active set, changing the current row. The cursor can only move forward in the active set. To return to a row that has already been fetched, you must reopen the cursor, then begin again at the first row of the active set.

If you want to change the active set, you must assign new values to the input host variables in the query associated with the cursor, then reopen the cursor. When MODE=ANSI, you must close the cursor before reopening it.

As the next example shows, you can fetch from the same cursor using different sets of output host variables. However, corresponding host variables in the INTO clause of each FETCH statement must have the same datatype.

If the active set is empty or contains no more rows, FETCH returns the "no data found" Oracle warning code to SQLCODE in the SQLCA (if MODE=ANSI then the optional SQLSTATE variable will also be set.) The status of the output host variables is indeterminate. (In a typical program, the WHENEVER NOT FOUND statement detects this error.) To reuse the cursor, you must reopen it.

5.4.4 Closing a Cursor

When finished fetching rows from the active set, you close the cursor to free the resources, such as storage, acquired by opening the cursor. When a cursor is closed, parse locks are released. What resources are freed depends on how you specify the options HOLD_CURSOR and RELEASE_CURSOR. In the following example, you close the cursor named EMPCURSOR:

EXEC SQL CLOSE EMPCURSOR END-EXEC.

You cannot fetch from a closed cursor because its active set becomes undefined. If necessary, you can reopen a cursor (with new values for the input host variables, for example).

When CLOSE_ON_COMMIT=NO (the default when MODE=ORACLE), issuing a COMMIT or ROLLBACK will only close cursors declared using the FOR UPDATE clause or referred to by the CURRENT OF clause. Other cursors are unaffected by a COMMIT or ROLLBACK and if open, remain open. However, when CLOSE_ON_COMMIT=YES (the default when MODE=ANSI), issuing a COMMIT or ROLLBACK closes all cursors. For more information, see "CLOSE_ON_COMMIT".

5.4.5 Using the CURRENT OF Clause

You use the CURRENT OF cursor_name clause in a DELETE or UPDATE statement to refer to the latest row fetched from the named cursor. The cursor must be open and positioned on a row. If no fetch has been done or if the cursor is not open, the CURRENT OF clause results in an error and processes no rows.

The FOR UPDATE OF clause is optional when you declare a cursor that is referenced in the CURRENT OF clause of an UPDATE or DELETE statement. The CURRENT OF clause signals the precompiler to add a FOR UPDATE clause if necessary. For more information, see"Mimicking the CURRENT OF Clause".

In the following example, you use the CURRENT OF clause to refer to the latest row fetched from a cursor named EMPCURSOR:

5.4.6 Restrictions

An explicit FOR UPDATE OF or an implicit FOR UPDATE acquires exclusive row locks. All rows are locked at the open, not as they are fetched, and are released when you commit or rollback. If you try to fetch from a FOR UPDATE cursor after a commit, Oracle generates an error.

You cannot use the CURRENT OF clause with a cursor declared with a join since internally, the CURRENT OF mechanism uses the ROWID pseudocolumn and there is no way to specify which table the ROWID relates to. For an alternative, see "Mimicking the CURRENT OF Clause". Finally, you cannot use the CURRENT OF clause in dynamic SQL.

5.4.7 A Typical Sequence of Statements

The following example shows the typical sequence of cursor control statements using the CURRENT OF clause and the FOR UPDATE OF clause:

5.4.9 The PREFETCH Precompiler Option

The precompiler option PREFETCH allows for more efficient queries by pre-fetching rows. This decreases the number of server round-trips needed and reduces memory required. The number of rows set by the PREFETCH option value in a configuration file or on the command line is used for all queries involving explicit cursors, subject to the standard precedence rules.

When used inline, the PREFETCH option must precede any of these cursor statements:

EXEC SQL OPEN cursor

EXEC SQL OPEN cursor USING host_var_list

EXEC SQL OPEN cursor USING DESCRIPTOR desc_name

When an OPEN is executed, the value of PREFETCH gives the number of rows to be pre-fetched when the query is executed. You can set the value from 0 (no pre-fetching) to 9999. The default value is 1.

Note:

The PREFETCH precompiler option is specifically designed for enhancing the performance of single row fetches. PREFETCH values have no effect when doing array fetches, regardless of which value is assigned.

5.5 Sample Program 2: Cursor Operations

This program logs on to Oracle, declares and opens a cursor, fetches the names, salaries, and commissions of all salespeople, displays the results, then closes the cursor

All fetches except the final one return a row and, if no errors were detected during the fetch, a success status code. The final fetch fails and returns the "no data found" Oracle warning code to SQLCODE in the SQLCA. The cumulative number of rows actually fetched is found in SQLERRD(3) in the SQLCA.